The invention relates to an apparatus and method for effectively mixing-in a fluid into a suspension, or fluid and is particularly adapted for the mixing of a treatment fluid into a comminuted cellulosic material suspension. The mixing of chemicals in pulp of medium consistency (i.e. about 5 to 15% solids consistency, preferably about 8 to 12% solids consistency) is particularly appropriate for the practice of the present invention.
Mixing apparatus is known in the prior art, such as shown in Swedish Pat. Nos. 172,981 and 354,789, wherein treatment fluid is passed through one or more movable outlet openings which form spiral-shaped "traces" in the pulp since the pulp has a more or less linear flow and the openings move in circles. While these prior art structures do effect mixing, there are disadvantages associated therewith in that a part of the pulp passes at a distance from the outlet openings and does not receive a sufficient amount of treatment medium. Furthermore, since the openings are provided in rotating arms or wings, a considerable resistance arises to the rotation of the fluid introducing apparatus thereby increasing power consumption, especially with medium consistency pulp and the like (i.e. about 5 to 15% consistency pulp).
According to the present invention, an apparatus and method are provided that effect an even distribution of treatment fluid or the like into a suspension with a minimum of power consumption. The effectiveness of the distribution and mixing depends upon many factors such as the pulp concentration, the fluid quantity to be added and the speed of reaction of the treatment fluid with the suspension, etcetera. According to the present invention it is possible to provide uniform distribution and mixing even when these factors vary widely.
Generally, the higher the concentration of solids in the suspension the more difficult it is to evenly distribute the treatment fluid. Also, the more quickly the treatment fluid reacts with the suspension the more important it is to distribute and mix the fluid in as quickly and evenly as possible. This is especially true with respect to the chlorine bleaching of paper pulp. In order not to dilute the pulp with an undesirable quantity of liquid, chlorine is usually added as a gas dispersed in a relatively small quantity of liquid. Chlorine has a quick initial reaction with the pulp and difficulties therefore arise in the even distribution of such a small quantity. According to the present invention, however, it is practical to provide even distribution of treatment fluid into the suspension even where the treatment fluid reacts quickly with the suspension, and the solids consistency of the suspension is high. For instance according to the present invention it is possible to effect chlorine bleaching of pulp of medium consistency (i.e. about 5 to 15%), with a minimum of power consumption and without any dilution.
According to one aspect of the present invention apparatus for effecting mixing of the fluid into a suspension is provided which comprises the following components: A conduit having at least a portion thereof that is substantially linear and substantially circular in cross-section. A substantially smooth-faced circular disc having an area substantially the same as, or slightly less than, the circular cross-section of the conduit. Means defining a through-extending opening in the disc, the opening having a small area compared to the area of the disc. Means for mounting the disc for rotation in the conduit substantially linear, substantially circular cross-section portion, about an axis of rotation substantially concentric with the conduit circular cross-section portion. And, means for introducing the fluid into the opening. The disc mounting means preferably comprises a hollow shaft while the means for introducing the fluid into the opening comprises the hollow shaft, a fluid passageway extending from the shaft to a source of fluid remote from the conduit, and means defining a cavity in the disc in operative communication with the both the hollow shaft and the opening. A motor or like means is provided for rotating the disc and shaft at a speed of about 300 to 1500 rpm.
According to another aspect of the present invention a method of mixing a fluid into a suspension is provided, particularly for mixing a treatment fluid (like chlorine or oxygen gas) into a medium consistency comminuted cellulosic material suspension (i.e. 5 to 15% solids consistency pulp). The method comprises the following steps: Defining a flow path for the suspension past an area of mixing fluid introduction. Moving the suspension along the flow path so that prior to and after the area of mixing fluid introduction it has a velocity below fluidizing velocity. Effecting turbulent fluidization, at any given time, of a small part of the suspension flowing in the flow path adjacent the area of mixing fluid introduction at the area of mixing fluid introduction. Evenly and uniformly introducing mixing fluid into the fluidized part of the suspension when fluidized; and progressively treating all parts of the suspension flowing in the flow path so at some time substantially the entire suspension flowing in the flow path will be subjected to turbulent fluidization and fluid introduction.
It is the primary object of the present invention to provide for the effective distribution and mixing-in of a fluid in a suspension with minimal power consumption. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.